We already posted about various situations that would be related to a fortress. Just to remind the definition of this tactic:
A fortress is a technique used in the endgame (but not always, read on) of chess to secure a draw when one side has fewer pieces than the other. This involves establishing a protected area that the opponent cannot breach, which may involve preventing their king from entering one’s own position or creating a zone that the opponent cannot force one out of.
For instance, our article Rook and king vs bishop and king shows a typical fortress pattern. The defending player will use it to guard the king with the bishop, eventually enforcing a draw.
Another intriguing usage of a fortress is called the Defensive Perimeter. A Defense Perimeter is distinct from other fortification techniques in that it can be established during the middlegame, even when numerous pieces are still present on the board. This technique is used to achieve a draw when one side is disadvantaged in material or otherwise. It involves creating a perimeter, which is primarily or entirely made up of a pawn chain, that the opponent cannot breach.
Have a look at the following example:
Fun fact: Chess engines usually fail on evaluating such situations. When I tried to import the final position to Stockfish 2.3.5 (maybe the future versions will become more advanced), it stubbornly claimed that White has 99.7% chance to win, even after analyzing to the depth of 62 moves.